1. Field of the Invention
The present invention relates to an optical information recording apparatus, as well as a method, for recording information in an optical information recording medium by using holography. The present invention also relates to an optical information playback apparatus, as well as a method, for reproducing or playing back information from an optical information recording medium by using holography. The present invention further pertains to optical information media suitable for use in the optical information recording apparatus and the method, as well as in the optical information playback apparatus and method of the present invention.
2. Description of the Related Art
In general, holographic recording technique for recording information in a recording medium by holography employs generation of interference fringes inside the recording medium by superposition of information light carrying image information and reference light so that the information is recorded in the form of the interference fringes. For the purpose of playing back, reference light is applied to the recording medium so as to be diffracted by the interference fringes, whereby the image information is reproduced in the form of the diffracted light.
In recent years, volume holography, in particular digital volume holography, has been developed to a practically usable extent in order to cope with demands for super-high density of optical recording, and is now attracting attention of the field concerned. Briefly, volume holography is a technique in which interference fringes are written three-dimensionally in a recording medium, through a positive use of the medium material in the thicknesswise direction of the same. According to this technique, it is possible to enhance the efficiency of diffraction by increasing the medium thickness, so that a greater storage capacity can be obtained through multiplex recording. Specifically, the above-mentioned digital volume holography is a technique which basically relies upon the volume holography stated above but deals with only digital patterns formed by binary-coded image information. Thus, the digital volume holography is a technique which is friendly to computer technologies. According to digital volume holography, any analog image information such as a picture is digitized and developed into two-dimensional digital patterns which are then recorded as the image information. Reproduction of the image information is conducted by reading and decoding the recorded digital pattern. It is possible to reproduce the original information with a high degree of fidelity despite any inferior SN (sound to noise) ratio during the playback, by the use of suitable technique such as differentiation detection and error correction through coding of the binary-coded data.
FIG. 1 is a schematic perspective view of a recording/playback system employed in a known digital volume holographic apparatus. This recording/playback system has a spatial optical modulator 101 which produces information light based on two-dimensional digital pattern information, a lens 103 which condenses the information light 102 and applies the condensed information light 102 onto a holographic recording medium 100, a reference light applying means (not shown) which applies reference light 104 to the holographic recording medium 100 in a direction substantially perpendicular to the information light 102, a CCD (Charge-coupled device) array 107 for detecting the reproduced two-dimensional digital pattern information, and a lens 106 which condenses the reproduced light 105 from the holographic recording medium 100 and applies the same onto the CCD array 107.
The operation of the recording/playback system shown in FIG. 1 is as follows. When recording information such as a picture, the system digitizes the original picture information into binary signals of 0 or 1 and two-dimensionally develops these binary signals to form two-dimensional digital pattern information. One batch of the two-dimensional digital pattern information will be referred to as "page data". The spatial optical modulator 101 selects the states of pixels, whether transmissive or non-transmissive, in accordance with page data #1, so as to form spatially-modulated information light 102 which is made to be incident to the holographic recording medium 100 through the lens 103. At the same time, reference light 104 is applied to the holographic recording medium 100 in a direction .theta.1 substantially perpendicular to the information light 102, so that the information light 102 and the reference light 104 are superposed inside the holographic recording medium 100 to form interference fringes which are recorded in the holographic recording medium 100. In order to achieve high diffraction efficiency, the reference light 104 is deformed into a flat beam by means of, for example, a cylindrical lens so that the interference fringes are recorded not only two-dimensionally but also in the direction of thickness of the holographic recording medium 100. For recording the next page data #2, another reference light 104 is applied in a direction .theta.2 which is different from the above-mentioned angle .theta.1 so as to be superposed on the information light 102, thus achieving multiplex recording in the holographic recording medium 100. Similarly, successive page data #3 to #n are recorded to achieve further multiplexing of the recording, through application of the reference light 104 at different angles .theta.3 to .theta.n. Each of the holograms thus recorded is referred to as a "stack". Thus, the structure shown in FIG. 1 has a plurality of stacks (stack 1, stack 2, . . . , stack m and so forth).
Any desired page data can be played back by applying the reference light 104 to the stack at the same angle as that applied when the page data was recorded. The reference light 104 is selectively diffracted by the interference fringes corresponding to the page data so that reproduced light 105 is produced. The reproduced light 105 is made to be incident to the CCD array 107 through the lens 106, so that two-dimensional pattern of the reproduced light is detected by the CCD array 107. The two-dimensional pattern of the reproduced light is subjected to processing reverse to the recording process by being decoded, whereby the information such as an original picture is played back.
The system shown in FIG. 1 enables multiplex recording of information in one holographic recording medium 100. In order to achieve super-high density of recording, it is important to exactly aim the information light 102 and the reference light 104. It is to be noted, however, the holographic recording medium 100 per se of the system shown in FIG. 1 lacks any means which would enable correct aiming of the light. Thus, the information light 102 and the reference light 104 can be aimed with respect to the holographic recording medium 100 could be achieved only through mechanical means which cannot provide sufficiently high degree of aiming precision. This impairs the removability (degree of ease of shifting the recording medium from one recording/reproducing apparatus to another for recording/playback purpose), and hampers random-accessibility, while making it difficult to achieve high density of the recording.
Another problem encountered by the system shown in FIG. 1 is that the size of the optical system is rendered large due to the fact that the optical axes of the information light 102, reference light 104 and detection light 105 are disposed at different spatial positions.